I have 3 PC in lan. One is my mail server, the others two are users's PCs. I have my ISP box on my lan that i use as gateway to go on the internet. I want to use my mail server as IMAP server for the two users and be able to send and receive mails (with POP3 and SMTP) to my mailboxes that i have on other sites (like gmail, msn, ISP ...)

I tried email routing, relay recipients and almost everything in ISPConfig without success.

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These settimgs are not relevant for relaying. the Relay settings are under System > Server config > Email.

According to the postfix error message, you tried to send a email without smtp authentication. So the first step you should do is to enable smtp aurhentication in your mail client, this is a client settings, not a setting of your server or in ispconfig.

I just find hard to believe that a "Perfect mail server" can't handle outgoing mails

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It works on more then hundred thousand servers worldwide that use this setup, so it should work on your servera s well And it does not matter if you use the server on a datacenter or your local lan.

I don't know if i found a bug but here's what i've found on my problem:

I can't send a mail to a mailbox whose domain if defined on my server but does not exist on it.

More specifically:
My ISP is Orange.fr. They were known as Wanadoo.fr years ago. I have mailboxes created with the @wanadoo.fr. If i want to fetch my mail from these boxes i have to define this domain in ISPConfig3 and set a fetchmail task.

By doing this i can get all my mail on my server. I can send emails to anybody but people that have email adresses ending by @wanadoo.fr. If i do that i have the "User unknown in virtual mailbox table". Sending mails to other users that end with @orange.fr works fine.

A few days ago i tried building a server "by hand". Mailing the same way was working (the "relayhost=" field in main.cf of postfix is used to send every unknown adresses on the server to an external relay). When i use ISPConfig3 it doesn't. That leaves two possibilities:
- I have not configured correctly ISPConfig3
- There is a bug in ISPConfig3

Thare is no such bug in ispconfig, just a misconfiguration on your sever. You can not define a domain of a external mail provider as local domain on your server and you dont need that to fetch emails. Thats not ispconfig specific, if you tell a mail server that it is authoritive for a domain name then it catches all email addresses of that domain an rejects unknown addresses. So you told the server that you are the owner of wanadoo.fr and the server acts like this.

Thre is no need for such a configuration, if you want to ftch emails from a existing wanadoo.fr account, then use the ftchmail function to get the emails from the pop3 or imap account.